I used to sail here (I think it will work on SW) Poddyshot Lane off Ministerly Rd. Flat water only. Walpole is good too on the inlet and Albany great for speed sailing.

Also try Crusoe beach road east of town. Further down the inlet and will probably be a little less gusty in the SW'er

I've just returned from that area. We camped at Windy Harbour and there are beautiful 'sailable' beaches, however very isolated and I didn't see any boat in 4 days, let alone alone windsurfers. I didn't take my gear as I was alone with my little one, but will definitely take it next time I go down there. The pictures below are from the beach they call the Cathedral.